U Ko Ni had just stepped off a plane and was standing curbside at the airport in Yangon, the largest city in Myanmar. The tall, gray-haired lawyer cradled his 3-year-old grandson while passengers around him spoke on their phones or climbed into taxis.

No one seemed to notice as a man in shorts and sandals sidled up behind Ko Ni, drew a 9-millimeter pistol inches from his head and pulled the trigger.

The fatal shooting not only silenced one of Myanmar’s most prominent legal experts, it exposed the dangers lurking below the surface of this former military dictatorship’s fitful transition to democracy.

In the old Myanmar — previously known as Burma and ruled by a junta for a half-century — political activists routinely disappeared into prisons or died in murky circumstances. Then in 2010, the military began ceding authority to civilians.

Pro-democracy icon Aung San Suu Kyi’s party won a parliamentary majority in 2015 elections, and last October the Obama administration lifted economic sanctions, formalizing Myanmar’s reentry into the global community.

But the military establishment still wields immense clout in this Southeast Asian nation of 53 million, authority enshrined in the constitution it passed in 2008 shortly before initiating reforms.

Ko Ni had spent the last several years drafting a new constitution that would have unwound many of the army’s powers, and his killing in January has shaken civil society leaders who see it as a warning to reformers.

“Those who did this did not tolerate progress,” said Myo Win, a Muslim activist who heads the Smile Education and Development Foundation, a nonprofit group in Yangon. “Of course, the rest of us are worried.”

Authorities say the assassination was a plot by three former military officers who hired an ex-convict to carry it out. The gunman and two other suspects have been arrested while the third, a retired army lieutenant colonel, remains at large.

Ko Ni’s grandson survived the shooting, but the gunman also killed a taxi driver who pursued him.

The home affairs minister, Lt. Gen. Kyaw Swe, said the suspects were motivated by “extreme patriotism” and angered by posts Ko Ni had written on social media. He did not specify the writings. But few figures represented a greater challenge to Myanmar’s establishment than the 63-year-old Ko Ni.

Besides advising Suu Kyi’s National League of Democracy party and defending the rights of his fellow Muslims in a Buddhist-dominated country, he was, behind the scenes, pushing a bold gambit to abolish the army-written constitution.

The document gives the army control of the entire civil service, and Ko Ni had told friends that as long as that provision remained in place, “the military is basically still running the country,” said Bertil Lintner, an author and commentator who has worked on Myanmar since the 1980s.

The army effectively holds veto power over any constitutional changes because a three-quarters majority in parliament is required to pass amendments, and one-quarter of seats are reserved for the military. Ko Ni thought he had found an opening: Scrap the constitution with a simply majority vote in parliament.

“There is nothing in the 2008 constitution that says it can’t be abolished with a single vote,” said Lintner, a longtime friend. “He was a constitutional expert, and very good at finding loopholes."

He had already devised the strategy that allowed Suu Kyi — the country’s most popular political figure — to lead the government after the 2015 elections. Sidestepping a constitutional provision that barred her from becoming president because her late husband had foreign citizenship, Ko Ni’s solution was to create the powerful post of state counselor, which sits above the president.

But Suu Kyi thought Ko Ni’s plan to do away with the constitution was “too provocative,” Lintner said. Although party officials said they remained committed to constitutional reform, many experts believe Ko Ni was uniquely qualified to lead the effort.

“With the loss of its chief technician and advocate, the constitutional reform process will almost certainly be stalled,” said Richard Weir, a fellow with the Asia division of Human Rights Watch.

Ko Ni often discussed his ideas in public forums and with journalists, including foreign reporters with whom he spoke in English. Last September, he confided in activist Myo Win that he felt threatened.

“Someone close to the military came and told him that he was their second-biggest enemy after Shwe Mann,” Myo Win said, referring to the former head of the military-backed Union Solidarity and Development Party who was ousted in 2015 for pushing constitutional reforms.

The following month, when Ko Ni spoke at a conference in the northeast city of Lashio, the audience of 200 or so attendees was joined by about 10 plainclothes officers from military intelligence.

“They always knew where he was,” Myo Win said.

But Ko Ni kept his fears from his family. His son Thant Zin Oo, a 29-year-old software engineer in Singapore, noticed the abuses hurled at him on social media sites but thought they were harmless.

“He did not mention anything that could cause us any concern, although there was online harassment constantly,” his son said.

Some of the vitriol he faced derived from Ko Ni’s faith. Muslims, who account for fewer than 5% of Myanmar’s population, have often been targeted by a surging Buddhist nationalism — particularly in western Rakhine state, where members of the Rohingya ethnic group are denied citizenship and have been systematically persecuted.

Ko Ni was not Rohingya but spoke out about the injustices faced by the group. He also criticized his own party for failing to field any Muslim candidates in the 2015 elections, an apparent effort to placate Buddhist extremists.

“I can think of many Muslim lawyers in Myanmar who very deliberately keep a much lower profile,” said Melissa Crouch, a senior lecturer at the University of New South Wales law school in Sydney, Australia, and an expert on Myanmar’s constitution. “He stood out.”

But over the last year, Crouch said, Ko Ni had come to believe that space for free speech was narrowing. Last November, when she invited him to speak in Yangon on a panel about constitutional issues — the type of event he usually welcomed — he refused.

“In Myanmar there are invisible lines and you never quite know when you’re going to step on them,” Crouch said. “And now that message has been very clearly understood.”

The day he was killed, he was returning from Indonesia, where he had traveled as part of a government delegation to share experiences of political reconciliation. Mya Aye, a Muslim activist who was part of the delegation, said Ko Ni had openly discussed the need for political reforms.

Both men had been targets of extremists before. In 2014, the National League of Democracy party had to cancel a public event after Buddhist monks protested the inclusion of the two men because they were Muslim.

“It’s never been safe for political activists in Myanmar,” Mya Aye said, “and now it is getting worse.”

Suu Kyi’s government has offered a mixed response to the assassination. The morning after his death, which made headlines worldwide, the state-run Global New Light of Myanmar newspaper ran the story on its inside pages. Suu Kyi waited one month before making a public statement, calling Ko Ni’s death a “deep loss” but stopping short of a full-throated appeal for justice.

Allies said she has been careful to avoid antagonizing military generals to maintain a working relationship — and because she might fear for her own safety.

“It seems she can’t do much,” Mya Aye said. “She might be thinking that to be vocal would cause unnecessary problems. But she needs to speak out for justice.”

Whether a plot to kill Ko Ni reached higher into the military establishment may never be known. Activists have already criticized the conduct of the investigation.

The police and army, which are running the probe together, waited three weeks to hold their first news conference. The home minister, Kyaw Swe, also raised eyebrows when he suggested without elaborating that Ko Ni’s “community” — a veiled reference to Muslims — might have killed him.

Ko Ni’s relatives say they won’t judge the investigation until it is over. Asked whether those responsible for his father’s killing would see justice, Thant Zin Oo said, “We have hope.”